This invention relates to a heating apparatus, particularly to a thermal imaging system employed in a crystal growing system which comprises an ellipsoidal reflecting mirror provided with two foci, and a heat source whereby a substance to be heated may be disposed on a line connecting these two foci to heat the substance by a concentrated infrared radiation from the heat source.
Such infrared radiation heating devices are utilized for the crystallization of various materials, such as refractory oxides by melting a material feed rod by using techniques based on either the floating zone principle or the pedestal principle, as disclosed, for example, in U.S. Pat. Nos. 3,761,677 and 3,817,710 or in IEEE Transactions on Magnetics, Vol. MAG-5, No. 3, September 1969, pages 285-289 or in NEC Research & Development No. 33, April 1974, pages 86-92, and Journal of Crystal Growth 39 (1977), pages 211-215. These U.S. Patents and literature show systems for growing single crystals by the floating zone method. In U.S. Pat. No. 3,761,677, the radiant energy is supplied by a pair of halogen lamps, whereas U.S. Pat. No. 3,817,710 and the literature show the use of a single lamp such as a halogen lamp or a xenon lamp for producing the radiant energy. For example, in a conventional heating apparatus using a single heat source the structure of the apparatus is simplified as shown in FIG. 1, in which the center of a xenon arc lamp or of a halogen lamp 1 forming the heat source is positioned at a first focus F1 of an ellipsoidal or prolate spherical reflecting mirror 2. A central junction between a substance 3 of a seed crystal and a polycrystalline material feed rod 5 to be heated, is positioned at a second focus F2 of the ellipsoidal or prolate spherical reflecting mirror 2, so that the radiated energy of the lamp 1 is concentrated at the second focus F2. Thus, the junction between the seed crystal and the material feed rod is partially heated to a high temperature to form a molten zone 4. In this case the heated substance at the second focus F2 is rotated at a fixed r.p.m. rate about an axis Y extending perpendicularly to a major axis X. In addition the substance is moved up or down in the direction of the axis Y at a very slow fixed rate.
When a high power xenon arc discharge lamp is used instead of the halogen lamp as a heat source 1 for the crystallization at high melting temperatures near 3000.degree. C., the concentration of radiation from the xenon lamp becomes too sharp due to the spot-like small heat source of the xenon lamp as compared to the halogen lamp. Such sharp concentration of the radiant energy produces a thermal image having a temperature distribution in the horizontal direction so that the maximum point is at the second focus F2 and the peaked portion decreases steeply in the vicinity of the second focus F2, as shown in a temperature distribution curve To of FIG. 3. Besides, although the heating apparatus must be so constructed that the vertical rotating axis Y of the heated substance 3 is positioned correctly at the second focus F2, it is somewhat difficult practically to let the rotating axis Y coincide with a center axis of a molten zone 4 in the junction of the seed crystal 6 and the polycrystalline feed rod 5 supported by upper and lower holding shafts, because the molten zone 4 in the vicinity of the second focus F2 is viscous due to the melting, whereby the center line of the rotating molten zone 4 tends to deviate from the axis Y. Therefore, a further distant side of the substance 3 from the second focus F2 in the horizontal direction is heated less, while the other opposite side of the substance 3 is strongly heated. As shown in FIG. 2(a), according to the invention it is desired to have a boundary line of the molten zone 4 which extends in parallel to the axis X for uniform heating. However, as shown in FIG. 2(b) on an enlarged scale relative to FIG. 1, the boundary lines between the molten zone 4 and the solid state portions of the polycrystalline feed rod 5 and the crystal seed 6 become inclined against the axis X in the conventional apparatus. Thus, prior art devices do not provide the desirable state as shown in FIG. 2(a). The facts shown in FIG. 2(b) cause an undesirable crystal growth. Moreover, when some parts along the circumference between the polycrystalline feed rod 5 and the molten zone 4 are left unmelted, the continuation of the single crystal growth cannot be maintained.